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7 stars at Amazon, 4. 6 stars at Home Depot, but that's only based on around 20 reviews across the two sites. The First Alert SA521CN 3ST Est. $40 is a hardwired version of the battery operated First Alert SA511CN2 3ST/SA511B interconnectable smoke detector. The two First Alert smoke detectors can be mixed and matched to create a smoke detector network that covers your premises. If you have a pre existing interconnected hard wired smoke detector network, a First Alert SA521CN 3ST sometimes sold as simply the First Alert SA521CN can be substituted for one of the existing units to act as a bridge so that when one smoke detector is activated, all smoke detectors on both networks will sound an alarm.